A high-frequency enhancement filter (edge enhancement filter) has conventionally been used to correct degradation in resolution caused by an optical system, such as a lens, and frequency characteristics after distortion correction.
Meanwhile, when an image is captured by using an optical system, depending on the optical system used, the resolution of the image decreases as one goes toward the perimeter, i.e., away from an optical center of the optical system. To this end, Japanese Patent Application Laid-open No. 2007-148500, for example, discloses use of a high-frequency enhancement filter. In this high-frequency enhancement filter, the filter coefficient is changed depending on the distance from the image-frame center so as to perform wider-range enhancement toward the perimeter of the image frame.
Considerably strong omnidirectional edge enhancement has been required to increase the strength of high-frequency enhancement (edge enhancement) in a perimeter portion of an image frame by using a conventional uniform, direction-independent filter. This produces a side effect that the noise is amplified greatly.
A fixed-coefficient filter can be implemented by means of bit shifters and adders. In contrast, the configuration for causing an image frame to have different filter coefficients at different portions of the image frame requires a large number of multipliers, resulting in massive circuit scale. This makes the configuration to be less appropriate in certain cases, such as use as on-vehicle equipment, where the configuration should operate even at low power.
The present invention aims to solve the above issues in the conventional technology.